The invention relates to a device for heating a meltable material.
Both in the metal-processing as well as the plastic-processing industry, a known method is to heat equipment used for melting, heat-retention or conveyance by disposing heating elements to abut the housings of this equipment. This method of heating is advantageous since transference of heat through direct contact, that is, through thermal conduction, is very efficient.
In practice, however, problems frequently occur due to the fact that, depending on thermal expansion conditions, gaps may be created between the heating elements and the individual housings, with the result that the thermal conductivity between the heating elements and the housings is considerably impaired. Additionally, depending on the temperature level reached, oxidation phenomena may appear on the housing or the surface of the heating elements, and these oxidation coatings may also significantly degrade thermal conductivity.
Aside from the fact that heating efficiency may be degraded due to the problems outlined above, the controllability of heating output, in other words, the ability to set a desired temperature within the heated material, may also be impaired by the above-mentioned problems such that often the entire process control, and thus the output from the downstream equipment, may be diminished.
U.S. Pat. No. 4,600,375 discloses a device for melting and conveying by thermal radiation. Here a resin is melted in a glass tube. Since the glass tube is transparent to the thermal radiation, in other words is permeable to it, the thermal radiation acts directly on the resin undergoing the heating. This device according to the species is not suited for processing metals.
The goal of the invention is to improve the device so as to enable this device to be used for melting metals as well.
The invention, in other words, offers the surprising proposal wherein a thermal radiation source is used as a heating device but the material for melting is not heated directly by using thermal radiation which passes through the conveyor channel, but indirectly. To achieve this, the conveyor channel does not completely absorb, or only partially absorbs, the thermal radiation, with the result that the heating of the material is effected not directlyxe2x80x94or not only directlyxe2x80x94by the thermal radiation but by thermal transfer from the conveyor channel to the material.
The absorption of radiation results in a smoothing out of the transfer of heat into the metal since an undesirably high percentage of the thermal radiation impinging on the metal, such as occurs during indirect radiant heating, may possibly be reflectedxe2x80x94with the result that the efficiency of the heating process is degraded. In addition, since the conveyor channel is not transparent to thermal radiation, it may readily consist of a material which, unlike glass, does not react with the metal or molten metal at high temperatures and wear out.
The heating of the conveyor channel itself is by radiant energy, an advantageous approach which avoids the above-mentioned disadvantages of crack formation or coating formation. The wavelength utilized may, for example, be the infrared or a wavelength closely adjacent to the infrared. The inertialess, because massless, transmission of heat in the form of thermal radiation additionally allows the temperature to be quickly and precisely adjusted. The heating elements may, for example, be appropriately take the form of a heating filament, discharge lamp, or be otherwise adapted to the specific operating conditions.
Since the conveyor channel absorbs thermal radiation, it heats up and thereby heats the transported material requiring heating. It is similarly possible to select a material for the conveyor channel which achieves both a partial transparency to radiation as well as a partial absorption of radiation, for example, so as to be able to utilize a material of especially high durability.
The heating means is located xe2x80x9coutsidexe2x80x9d the conveyor channel, meaning that there is a separation between the heating means and the material to be transported and heated. However, this does not mean, for example, that given a circular geometry for the device the configuration must necessarily extend radially outward from the conveyor channel.
The conveyor channel may be advantageously designed as a tube which completely surrounds the conveyed material such that the material is shielded from undesirable extraneous effects. The radiant heating means may be advantageously arranged to completely surround such a tube so as to act on the conveyor channel from all sides, thus producing an intensive, encompassing and uniform heating of the heated material. Compared with a similarly conceivable geometry for a conveyor channel with an annular cross-section in which the heating means is located inside this annular cross-section, a configuration with a heating means acting radially from outside results in a larger surface on which the radiant heat can act such that the transfer of a higher heat output is promoted. Depending on requirements, the heating means may be located both inside as well as outside this annular cross-section to produce an especially intense action of the radiant energy.
A partition wall may be advantageously provided between the heating means and the conveyor channel, which partition wall is transparent to the thermal radiation, that is, permits nearly unobstructed heating of the conveyor channel, but which prevents convection, that is, the transfer of heat by the movement of air or gas. This approach creates a zone which is maintained at as cool a level as possible in which to locate the radiators, that is, the sources of thermal radiation, so as to reliably prevent them from overheating. At the same time, the partition wall permits the thermal intensity in the region of the conveyor channel to be maintained at as high a level as possible.
Another function of the partition wall may be to employ a specific atmosphere within the region of the conveyor channel or in the region of the radiation sources, which atmosphere is advantageous for the mechanical or thermal stability of the conveyor channel or the radiation sourcesxe2x80x94with the result that these components will exhibit an especially long useful life during operation.
With the aim of keeping the temperature level in the region of the conveyor channel as high as possible, and thus prevent heat losses, the partition wall may have the advantageous feature of being transparent to radiation on one side and reflecting in the other direction, these characteristics being adapted to the specific wavelengths at which the thermal radiation, on the one hand, emanates from the heat sources and, on the other hand, is reflected by the conveyor channel or heated material. In this way, the thermal radiation is allowed first to pass essentially unobstructed through the partition wall while the reflected radiation is then essentially retained by the partition wall and reflected into the region of the conveyor channel or material.
The heating means may be advantageously encircle the conveyor channel in an annular or helical pattern. Unlike other, for example, meander-shaped patterns, this design results in an essentially homogenous distribution of the thermal radiation, and thus in a homogenous distribution of temperature.
Heating may be advantageously differentiated by section in order to adjust the heating requirement over the length of the conveyor channel, for example, to allow first for melting zones and then heat-retention zones for the already-heated material within the conveyor channel.
The conveyor channel may be advantageously located completely within a closed housing. This feature permits a specific atmosphere to be employed within this housing, for example by evacuating the housing using inert gas or the like, which approach ensures the optimum stability of the conveyor channel, the term stability being understood to refer especially to the surface properties of the conveyor channel, such that these thermal-radiation-absorbing properties may be maintained as reliably as possible while ensuring a long useful life for the conveyor channel. By employing a suitable material, this housing may simultaneously include the functions of the above-mentioned partition wall.
The partition wall may be advantageously fabricated from quartz glass since this material offers an advantageously high temperature stability and simultaneously an advantageously high transparency to thermal radiation. In order to provide the desired reflection characteristics, this quartz glass may be surface-treated, for example, provided with a coating.
Reflectors may be advantageously provided which reflect the thermal radiation from the heat sources to the conveyor channel so as to minimize heat losses and enhance the intensity of the heat acting on the conveyor channel.